Catalyst for a n-butane oxidation to maleic anhydride

ABSTRACT

An improved vanadium-phosphorus mixed oxide is prepared by a unique method employing an organic medium. The substantial intrinsic surface area and the microcrystalline structure of the new oxide result in advantageous activity and selectivity effects in the catalyzed vapor phase oxidation of n-butane to maleic anhydride. These oxides are also useful for the production of acid anhydrides from suitable hydrocarbon feeds.

0 ilmted States Patent 11 1 1111 3,864,280

Schneider Feb. 4, 1975 CATALYST FOR A N-BUTANE OXIDATION 3,226,337 12/1965 Riemenschneider et al. 252/435 o MALEIC ANHYDRIDE ierr err [75] Inventor: loi zfild A- Sch eide Berkeley, 3,385,796 5/1968 Kerr 252/435 x [73] Assignee: Chevron Research Company, San Primary Examiner-Patrick P. Garvin Francisco, Calif. Attorney, Agent, or Firm-G. F. Magdeburger; John Filed: Oct. 1972 Stoner, Jr., T. G. DeJonghe [21] Appl. No.: 298,074 [57] ABSTRACT Related Apphcamn Data An improved vanadium-phosphorus mixed oxide is 62 N 263 883 J 19 1972 1 Dmslo" of 1 1 prepared by a unique method employing an orgamc medium. The substantial intrinsic surface area and the [52] Cl 252/435 252/437 26O/346'4 microcrystalline structure of the new oxide result in 269/3468 advantageous activity and selectivity effects in the cat- Ilit. alyzed vapor phase Oxidation of n butane to maleic Fleld of Search anhydride These oxides are also useful for the p Referen es Cited duction of acid anhydrides from suitable hydrocarbon feeds.

16 Claims, N0 Drawings CATALYST FOR A N-BUTANE OXIDATION TO MALEIC ANIIYDRIDE CROSS-REFERENCE TO RELATED APPLICATION BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an improved hydrocarbon oxidation catalyst especially suitable for use with saturated hydrocarbon feeds. More particularly, it relates to improved vanadium oxide-phosphorus oxide composites. Still more particularly, it relates to the production of maleic anhydride from n-butane in a vapor phase process employing the foregoing catalyst.

2. Prior Art Description It is known in the art that n-butane can be used as a feed for the production of maleic anhydride. n-Butane is available from petroleum refinery streams and is a relatively inexpensive feed for a maleic process. Nevertheless, there is little or no use of n-butane as a process feed for the production of maleic anhydride. Why is this? It appears that known catalysts for the partial oxidation of n-butane are unsatisfactory. A very active catalyst is required for the oxidation. Yet this very characteristic tends to be incompatible with the requirement that the oxidation be a selective partial oxidation rather than a total oxidation to carbon oxides. That is to say, presently known catalysts for the oxidation of n-butane to maleic anhydride have, in general, relatively poor selectivities.

Representative descriptions in the art which relate to the production of maleic anhydride from n-butane include US. Pat. No. 3,293,268.

THE INVENTION A catalyst having excellent activity and improved selectivity in the oxidation of n-butane to maleic anhydride has now been found. It is a crystalline composite or complex of vanadium oxide and phosphorus oxide which has:

1. an intrinsic surface area of the mixed oxide per se in the range from about 7 to 50 square meters per gram;

2. a phosphorus to vanadium atomic ratio in the range 0.9-1.8 to 1, respectively;

3. a vanadium oxide component having an average valence for vanadium in the range plus 3.9 to 4.6; and

4. a phosphorus oxide component having an average valence for phosphorus of about plus 5.

By intrinsic surface area of the mixed oxide, as used in this description, is meant the surface area of the material itself, i.e., per se, and in the absence of a support or carrier.

METHOD OF PREPARATION The novel crystalline compositions herein are prepared by precipitation of a vanado-phosphato mixed oxide complex from an essentially organic solvent medium. A minor amount of water may be present in the medium and indeed water or water-forming components bound in the precipitate are believed to be required in order to obtain a catalyst having a satisfactory selectivity and activity. The presence of a gross amount of water in the medium during the crystal-forming precipitation prevents the formation of the present novel mixed oxide catalysts having a relatively high intrinsic surface area. The precipitation is effected by evaporation of the organic solvent. The resulting crystalline solid is freed of solvent, and is then activated by heating in air for use as an oxidation catalyst. The activation procedure is required to condition the precipitated and dried mixed oxide (i.e., it is a catalyst precursor) for use as an oxidation catalyst. During the activation bound water (apparently water of hydration), or a mixture of water and organic solvent is evolved and a crystal phase change occurs.

EMBODIMENT into the isobutanol and for the exclusion of water vapor. For each formula weight in grams of the vanadium pentoxide, about 1 liter of the isobutanol solvent is used. The resulting slurry is then stirred and the vanadium pentoxide is dissolved in the isobutanol by passing a stream of anhydrous hydrogen chloride gas into the slurry while maintaining the temperature between 30 and 40C. The resulting solution is red-brown colored and nearly saturated with hydrogen chloride gas. It is ready for mixing with a solution of orthophosphoric (H PO acid in isobutanol.

The orthophosphoric acid solution is prepared by adding crystalline orthophosphoric acid to isobutanol in a glass lined vessel large enough to accommodate the solution of vanadium oxide prepared as described above and the phosphoric acid solution. For each mol of the acid a volume of about 100 milliliters of isobutanol is desirably used. Sufficient orthophosphoric acid is charged to the vessel to yield, upon the addition of the red-brown solution of dissolved vanadium oxide, a phosphorus to vanadium atomic ratio of 1.2. The vessel should be fitted for stirring, temperature control, reflux, the introduction of the red-brown solution and for the exclusion of water vapor.

After the introduction of the red-brown solution to the phosphoric acid solution in the second vessel, the temperature of the resulting solution is increased to the reflux temperature, i.e., about ll0C., and is maintained for about 1.5 hours. Evidence ofa reaction is notable in that the solution changes in color, usually to a greenish-brown. Thereafter, the reflux condenser is removed and isobutanol solvent is distilledfrom the reaction mixture. During the heating above at reflux and subsequently during distillation, hydrogen chloride gas evolves from the solution and is vented.

As the heating is continued and the volatiles, mainly isobutanol and hydrogen chloride, are evolved, the color of the solution continues to change; transitorally it assumes various shades of green or blue, the colorations associated with vanadium in the plus 4 valence (oxidation) state. Finally, after about two-thirds of the solvent has been evolved, the solution is colored a greenish-blue. Some light blue precipitate is usually present in the concentrate at this time. The remainder of the volatiles are conveniently removed by placing the concentrate in a loosely covered glass vessel in a ventillated oven maintained at C. Drying is continued until resulting precipitated solid reaches a substantially constant weight.

The dried precipitate is a uniform, very dark greengray solid which after activation has a surface area (BET-Method) of about 23 m lgr. For fixed bed use it is broken up and sieved to a 20-28 mesh (Tyler Screen) size.

For the activation the following schedule is carried out:

1. Heat the precursor to 380C. in a stream of air flowing at 1.5 volume/volume/minute. The heat input should yield a rate of temperature increase of 3C. per minute.

2. Maintain the 380C. temperature and the same air flow rate for 2 hours.

3. Increase the temperature from 380C. to 480C. at the 3C. per minute rate of increase while passing an air-butane mixture, 1.5 volume percent of butane in air, through the bed 10 inch by 0.5 inch diameter tube) at a flow rate of 1.5 vo1ume/volume/- minute.

4. Maintain the precursor at 480C. for about 16 hours while continuing the air-butane flow rate as before.

5. Reduce the temperature from 480C. to 420C. and then increase the air-butane flow rate to a VHSV of 1,000 hr. (17 vol/vol/min.).

6. Finally adjust the temperature upward or downward as required until the butane conversion is 90 percent.

Usually the catalyst performance stabilizes after a short run of from 6 to 36 hours. The activated catalyst has a standard activity (see discussion below) which is generally below 400C., usually 1 in the range 365390C. and a surface area (BET) of about 23 square meters per gram. This catalyst usually produces 105 kilograms of maleic anhydride per 100 kilograms of n-butane fed to the reactor or 1 17 kilograms of maleic anhydride per 100 kilograms of n-butane converted.

STANDARD CATALYST TEST In order to obtain a reliable comparison of oxidation catalysts herein, a standard test was required and developed. Ten milliliters of 20/28 mesh mixed oxide was charged to a one-half inch stainless steel reactor. After activation, performance was measured at a space velocity (volume at C. and l atm/volume/hr) of 1000 hr." using a feed of 1.5 volume percent n-butane in air. The catalyst was left onstream until its performance changed very little over a 24 hour period.

As the index of activity,.1 define standard activity as that temperature required for 90 percent conversion under the above conditions. Yield is defined as pounds of maleic anhydride produced per 100 pounds of hydrocarbon fed, and selectivity as pounds of maleic anhydride produced per 100 pounds of feed converted.

SURFACE AREA Depending upon variations in the organic solvent system employed and in some degree upon the activation procedure, the surface area [BET Method cf., H. Brunaur, P. H. Emmett, and E. Teller, JACS., Volume 60, Page 309 (1938)] ranges from 7 to 50 square meters per gram and higher. The preferred mixed oxide catalysts have surface areas in the range from to 50 m /gr and higher. In general, the higher the intrinsic surface area, the more active is the catalyst, and the lower is the temperature at which the catalyst is satisfactory for use in the oxidation of a saturated hydrocarbon feed.

P/V ATOMIC RATIO The precipiated mixed oxides herein should have an atomic ratio, phosphorus to vanadium, which in general is in the range from 0.9-1.8 to 1, preferably l.0l.5 to 1, respectively.

AVERAGE VALENCE OF VANADlUM COMPONENT The average valence of the vanadium in the activated mixed oxides of the invention is, in general, in the range from 3.9 to 4.6. Better results are believed to obtain when the average valence is in the range 4.1 to 4.4.

B-PHASE CONTENT TABLE 1 Line Position d (Angstrom) 2,6, Degrees Intensity, 1

The dimensions of the unit cell for B-phase, as obtained from the complete powder X-ray diffraction data, are a b 19.2 A and c 7.8 A. The crystalline phase is of hexagonalstructure.

The phosphorus-vanadium mixed oxides which have a B-phase content of at least 25 percent and an intrinsic surface area in the range above 10 m /gr. exhibit good activities and selectivities in the vapor phase partial oxidation of saturated hydrocarbon feeds. Consequently, these composites are preferred. Those having a B- phase content in excess of 50 percent exhibit, in general, excellent activities and selectivities and are most preferred. The relative amount of B-phase in a given composite is conveniently measured by the method of the National Bureau of Standards (Reference, United States Department of Commerce, N.B.S., Monograph 25, Section 6, Page'3). 1n the method a-alumina is used as an internal standard, and the x-ray diffraction pattern for a 5:1 (weight) mixture of mixed oxide and a-alumina is obtained. The ratio of the intensity (1) of the d 3.9 A line of the mixed oxide to the d= 2.085 A line of the a-alumina standard is a measure of the relative amount of the B-phase content for a given composite. With a sample of mixed oxides having approximately 100 percent B-phase, the ratio of intensities is 3.1. If the B-phase content is 25 percent, then the ratio is approximately 0.8.

The following examples further illustrate the invention.

The mixed oxide compositions herein may be prepared by a variety of reagents using a substantially ormixed oxides having a high intrinsic surface area. Examples 15-19 illustrate the effect of water upon the catalyst surface area and/or activity. Examples 20 and 21 are examples of catalysts prepared in the absence of ganic medium. Table ll below includes a number of 5 a solvent. represemanvi reactzm i 'z r in Table III below are listed the preparative methods of T; crush mlxe F n e employed for the several combinations of reactants l 'P es ustlratet i g a 1 listed above and a designator. The two designators, a resemat've orgamc so vents or t e pm ca ital letter for reactants, and a numeral for method,

are used to tdentif the mixed oxides in Table IV below.

TABLE ll REACTANTS DESIGNATOR I TABLE I l. VOCI 1.2 H PO X H O A 2.'/2VO,+l.2POCl,+XHO B 3. k i l: 2 "3P0: X 220+ HCI C METHOD OF PREPARATION DESIGNATOR (excess) l he paring a solution, then boiling off l) Z Q 5 E 6.531% E the solvent to leave a solid mass;

' i H 6 k g 20 2 Preparlng a solution, boiling off part (2) 6 v 0' a V O I 2 H PO F of thesolvent and collecting the i i H 0 resulting prempttate. I 7 V O H (857) G 3. M x ng the reactants w thout solvent. (3) 2 5 n 4 4. Mixing the reactants without solvent (4) and fusing at 900C. for 2 hours. "Gascous HCl bubbled into reactants until dissolution of V103. v

TABLE [V STANDARD MOLS CATALYST PERFORMANCE ACTIVATED CATALYST WATER SURFACE OXIDATION EX. REACTANTS ADDED CONV. TEMP. YIELD AREA STATE OF B-PHASE N0. AND X" SOLVENT" C. WT. (BET,m /g) VANADIUM METHOD 1 A-l 1.2 THF 90 430 I04 19 4.1 88 2 13-1 3.6 'l'HF 90 408 98 18 4.2 83 3 A-l O THF"'" 90 420 96 22 4.] 90 4 C-1 0 lsobutanol 90 374 105 23 4.2 73 5 8-1 3 .6 1,2-Dimethoxy- 90 452 97 12 4.4 42

ethane 6 B-l 3.6 do 90 422 89 15 7 B-l 3.6 do. 90 404 93 21 4.1 8 C-1 0 Methanol 90 468 90 8 4.3 8| 9 C-1 0 Acetic Acid 90 465 76 10 to 0-1 3.6 1,2-Dimethoxy- 90 472 73 l l 27 ethane l l C-l 0 Acetic Acid 90 446 73 l5 12 F-] 0 ClCH,CH,CI: 90 446 76 10 Methanol1z2: 1 l3 B-l 3.6 Acetone 90 553 25 I5 4.6 14 F-l 0 Ethyl Acetate: 85 SH) 54 4,1

EthanolzAcetic Acid 10:6:1 EFFECT OF WATER l5 F-l O Acetic Anh dride I6 510 5 4.6 l6 E- 0 Acetic Aci 38 510 23 I6 3.9

ofl) l7 E-2 2.0 Acetic Acid 90 399 92 15 4.2 18 C-1 5.0 Acetic Acid: 90 424 83 ll H O 16:3 l9 F-l 20 Propionic Acid: 49 510 42 5 H O 2: 20 0-3 None 90 532 3 4.l 40 21 0-4 None 48 536 17 4.5

NOTES "500 mls of solvent used per gram atom of phosphorus. Solvent ratios are by volume.

Based on butane fed.

"l'etrahydrofuran.

"33 Wt. 11 maleic acid. based on V,O added.

'66 Wt. bis-t2-methoxyethyl)ether. based on V,O,. added. "'2 mols puraformuldchydo per mol V O "4 moln acetic unhydride per mol V,O,.

""l mul of henznldchyde added per mol of VJ) "'(utulyxt performance determined at a space velocity of L000 hrsi. (STP) and using L5 volume percent of n-hutunc in air.

"An mcunurcd by x-ruy diffraction.

Examples l-13 illustrate that a mixed oxide complex of vanadium oxide and phosphorus oxide having a relatively high intrinsic surface area, for example in the range above 7 square meters per gram, and a B-phase content above percent has an excellent activity and selectivity as a catalyst for the partial oxidation of nbutane to maleic anhydride.

EXAMPLES 22-28:

In the manner described in the preferred embodiment, a series of mixed oxide complexes was prepared, activated, and tested except that the phosphorus to vanadium (P/V) atomic ratios of the complexes were varied as follows: 0.9, 1.0, 1.1, 1.2, 1.3, 1.5, and 1.8. The resulting comparative data for the activated catalysts is listed below in Table V.

tion or loss of activity. Estimated useful lives for these catalysts exceed 8 months and probably are 16 months or more.

The fixed bed or fluid bed process conditions normally employed for the partial oxidation of a hydrocarbon feed are, in general, satisfactory for use with the high surface mixed oxides of the invention as follows:

CONDITION RANGE Temperature, C. Pressure, atm. Contact time, sec.

300-500, preferably 325490 0.5-l0, preferably l-5 0.05-5, preferably 0.1-2,

and a feed mixture, air plus hydrocarbon, or oxygen- "By x-ray diffraction analysis. 'Based on butane fed at the standard activity temperature.

These examples demonstrate that both the yield and containing gas plus hydrocarbon which is outside the the activity of the catalyst composites are directly related to the B-phase content.

EXAMPLES 29-35:

Using a catalyst prepared in the manner described in the embodiment, and a space velocity of 1000 hr", and a feed of 1.5 volume percent hydrocarbon in air, other hydrocarbon feeds were oxidized. The feeds and results obtained are listed in Table VI below.

t l) 31% phthulic plus 8% maleic anhydride.

The examples in Table \(I and the above-described examples in which an n-butane feed was oxidized to maleic anhydride demonstrate that the novel catalyst compositions herein are useful as catalysts for the partial oxidation of a suitable hydrocarbon feed with molecular oxygen for the production of maleic or phthalic anhydride. Suitable hydrocarbon feeds iryrzlttde xylene and saturated or monoor diolefinic acyclic or cyclic hydrocarbons which have a carbon atom content in the range 4 to 10 and a linear chain of at least 4 carbon atoms.

The high surface phosphato-vanado mixed oxide complexes herein exhibit, in general, long catalyst lives and excellent retention of selectivity. After the initial breaking in of the activated catalyst, long periods of continuous use are indicated. Thus, over a period of 1,200 hours on stream, a representative catalyst has been used without evidence of appreciable deactivaexplosive range.

In view of the exceptional activities of the high surface mixed oxides of the invention, they are especially effective for use as catalyst for the partial oxidation of n-butane. The oxidation conditionsin this case desirably include a temperature in the range 350-490C., preferably 375475C., and the other condition, as noted above. n-Butane-air feed mixtures which contain an amount of n-butane in the range 0.5 to 1.8 volumes per volume of air are preferred feed mixtures for use in fixed bed reactors. In the case of fluid bed reactors, a more concentrated feed stream is satisfactory and the range of the hydrocarbon to air volume ratio may be as much as 0.5-l0 to 100.

The mixed oxides of the invention may be sized as desired in the usual manner, grinding, screening and the like, and employed. They may also be comminuted, slurried in a suitable liquid medium, and extruded or pelleted as desired. Similarly, they may be comminuted, slurried in a suitable liquid medium with or without an ordinary binding agent and shaped as desired, for example in spheres, or disposed upon a suitable inert support such as alumina, titania, silicon carbide, silica kieselguhr, pumice, and the like. Alternatively, the mixed oxide may be disposed upon an inert carrier by adding the carrier to a concentrate of the oxides in an essentially organic medium and evaporating the organic solvent.

Clearly, modifications and variations of the invention as hereinbefore set forth and exemplified may be made without departing from the sense thereof. Therefore,

only such limitations should be imposed as are indicated in the appended claims.

I claim:

1. A crystalline phosphorus-vanadium mixed oxide hydrocarbon oxidation catalyst composition containing pentavalent phosphorus, vanadium and oxygen, said vanadium having an average valence in the range from about plus 3.9 to plus 4.6, said oxide having a phosphorus to vanadium atomic ratio in the range from about 0.9-1.8 to 1, and an intrinsic surface area in the range from about 7 to 50 square meters per gram.

2. A composition as in claim 1 wherein:

a. the average valence of the vanadium is in the range from about 4.1 to 4.4;

b. the phosphorus to vanadium atomic ratio is in the range 1.0-1.5 to 1; and

c. the intrinsic surface area is in the range to 50 square meters per gram.

3. A composition as in claim 1 wherein said mixed oxide has a B-phase content in excess of about 25 percent.

4. A composition as in claim 2 wherein said mixed oxide has a B-phase content in the range from 50 to 100 percent.

5. A composition of claim 1 disposed upon an inert support.

6. A composition in accordance with claim 1 wherein the intrinsic surface area is in the range 10 to 50 square meters per gram.

7. A crystalline phosphorus-vanadium mixed oxide composition consisting essentially of pentavalent phosphorus, vanadium and oxygen, said vanadium having an average valence in the range from about plus 3.9 to plus 4.6, said oxide having a phosphorus to vanadium atomic ratio in the range from about 0.9-1.8 to 1, and an intrinsic surface area in the range from about 7 to 50 square meters per gram.

8. A composition as in claim 7 wherein:

a. the average valence ofthe vanadium is in the range from about 4.1 to 4.4;

b. the phosphorus to vanadium atomic ratio is in the range 1.0-1.5 to l; and

c. the intrinsic surface area is in the range 10 to 50 square meters per gram.

9. A composition as in claim 7 wherein said mixed oxide has a B-phase content in excess of about 25 percent.

10. A composition as in claim 8 wherein said mixed oxide has a B-phase content in the range from 50 to 100 percent.

11. A composition of claim 7 disposed upon an inert support.

12. A composition in accordance with claim 7 wherein the crystals of said composition have a unit cell of the dimensions a b about 19.2 A and c about 7.8 A.

13. A crystalline phosphorus-vanadium mixed oxide composition consisting essentially of pentavalent phosphorus, vanadium and oxygen, said vanadium having an average valence in the range from about plus 3.9 to

plus 4.6, said oxide having a phosphorus to vanadium atomic ratio in the range from about 0.9-1.8 to 1, and an intrinsic surface area in the range from about 10 to 50 square meters per gram, said crystals exhibiting an X-ray powder diffraction pattern (CuKa) when mixed with a-alumina in a 5 to 1 weight ratio of the mixed oxide to a-alumina in which the ratio of intensities of the d= 3.9 A line to the d= 2.085 A line of the mixture is at least about 0.8.

14. A composition as in claim 13 wherein said ratio of intensities is in excess of about 1.6.

15. A composition as in claim 13 wherein said ratio is in excess of about 1.6 and said intrinsic surface area is about 23 square meters per gram and said phosphorus to vanadium atomic ratio is about 1.2.

16. A composition as in claim 15 wherein said phosphorus to vanadium atomic ratio is in the range 1.0-1.5 to 1 and said average valence of the vanadium is in the range from about 4.1 to 4.4. 

2. A composition as in claim 1 wherein: a. the average valence of the vanadium is in the range from about 4.1 to 4.4; b. the phosphorus to vanadium atomic ratio is in the range 1.0-1.5 to 1; and c. the intrinsic surface area is in the range 10 to 50 square meters per gram.
 3. A composition as in claim 1 wherein said mixed oxide has a B-phase content in excess of about 25 percent.
 4. A composition as in claim 2 wherein said mixed oxide has a B-phase content in the range from 50 to 100 percent.
 5. A composition of claim 1 disposed upon an inert support.
 6. A composition in accordance with claim 1 wherein the intrinsic surface area is in the range 10 to 50 square meters per gram.
 7. A crystalline phosphorus-vanadium mixed oxide composition consisting essentially of pentavalent phosphorus, vanadium and oxygen, said vanadium having an average valence in the range from about plus 3.9 to plus 4.6, said oxide having a phosphorus to vanadium atomic ratio in the range from about 0.9-1.8 to 1, and an intrinsic surface area in the range from about 7 to 50 square meters per gram.
 8. A composition as in claim 7 wherein: a. the average valence of the vanadium is in the range from about 4.1 to 4.4; b. the phosphorus to vanadium atomic ratio is in the range 1.0-1.5 to 1; and c. the intrinsic surface area is in the range 10 to 50 square meters per gram.
 9. A composition as in claim 7 wherein said mixed oxide has a B-phase content in excess of about 25 percent.
 10. A composition as in claim 8 wherein said mixed oxide has a B-phase content in the range from 50 to 100 percent.
 11. A composition of claim 7 disposed upon an inert support.
 12. A composition in accordance with claim 7 wherein the crystals of said composition have a unit cell of the dimensions a b about 19.2 A and c about 7.8 A.
 13. A crystalline phosphorus-vanadium mixed oxide composition consisting essentially of pentavalent phosphorus, vanadium and oxygen, said vanadium having an average valence in the range from about plus 3.9 to plus 4.6, said oxide having a phosphorus to vanadium atomic ratio in the range from about 0.9-1.8 to 1, and an intrinsic surface area in the range from about 10 to 50 square meters per gram, said crystals exhibiting an X-ray powder diffraction pattern (CuK Alpha ) wHen mixed with Alpha -alumina in a 5 to 1 weight ratio of the mixed oxide to Alpha -alumina in which the ratio of intensities of the d 3.9 A line to the d 2.085 A line of the mixture is at least about 0.8.
 14. A composition as in claim 13 wherein said ratio of intensities is in excess of about 1.6.
 15. A composition as in claim 13 wherein said ratio is in excess of about 1.6 and said intrinsic surface area is about 23 square meters per gram and said phosphorus to vanadium atomic ratio is about 1.2.
 16. A composition as in claim 15 wherein said phosphorus to vanadium atomic ratio is in the range 1.0-1.5 to 1 and said average valence of the vanadium is in the range from about 4.1 to 4.4. 